IR-Based Monthly Rainfall for the GPCP
Online FTP Access to Arkin and Janowiak GPI Data
- Summary
Sponsor
Original Archive
Future Updates
Data Set Description
- Data Characteristics
Data Format
- Sample Software
References
Data Access
- Anonymous FTP
- Points of Contact
This README file describes the Arkin and Janowiak GOES Precipitation
Index (GPI) data, including information on the file structure, pertinent
scientific references, and contact information for obtaining the data.
This data set is one of the Global
Precipitation Climatology Project (GPCP) products. The GPI data
are infrared-based monthly rainfall estimates, which form an
intermediate product of the GPCP, a World
Meteorological Organization/World Climate Research Program
(WMO/WCRP) effort.
The distribution of this data set is being funded by NASA's Earth Science Enterprise progra
m. The data are not copyrighted; however, we request that
when you publish data or results using these data, please acknowledge as
follows:
The authors wish to thank the Distributed Active Archive Center (Code 610.2) at the Goddard Space Flight Center, Greenbelt, MD, 20771, for
distributing the data; and the science investigators, Drs. Phillip
Arkin and John Janowiak of the Climate Analysis Center, NOAA,
Washington, D.C., for producing these data products. Goddard's
contribution to these activities was sponsored by NASA's Mission to
Planet Earth program.
This data set was originally archived at the Marshall Space Flight
Center. It was moved to the Goddard Space Flight Center Distributed
Active Archive Center (GSFC DAAC) in the fall of 1996.
The Goddard DAAC will update this data set as new data are processed and
made available by the data producers.
The monthly rainfall estimates are based on infrared radiometer
measurements which form an intermediate product of the WMO/WCRP effort
(See Janowiak and Arkin (1991) for a general
description of the method). The rainfall data file consists of 2.5
degree latitude by 2.5 degree longitude gridded monthly accumulations
of estimated rainfall. Richards and Arkin (1981)
determined that this spatial resolution yielded the highest correlation
between fractional coverage of "cold" clouds and observed rainfall.
The primary source of information over the tropical oceans is infrared
(IR) data from polar-orbiting and geostationary satellites. The
estimates are generated using a simple cloud top
temperature-thresholding algorithm. This method of using
satellite-observed cloud top brightness temperatures to infer areas of
deep convection was demonstrated by Arkin (1979)
using data from the Global Atmosphere Research Program - Atlantic Tropical
Experiment (GATE) (See Hudlow and Patterson
(1979)). The estimates are valid for the tropics and the warm season
extratropics, and will overestimate rainfall in regions of persistent, thin, high
cirrus.
A reduction correction has been applied to the GPI estimates produced
from geostationary IR coverage at large zenith angles (e.g., 33
degrees). A systematic bias was found by comparing GPI estimates
computed separately from each geostationary satellite's IR data in
overlap regions. The satellite with a larger zenith angle yielded a
larger amount of clouds colder than the GPI's temperature threshold of
235 K, compared with the amount of clouds derived from satellites with
smaller zenith angles. This correction at least partially removed this
bias.
Data Characteristics
| · Parameters: | Accumulated surface precipitation |
| · Units: | mm/day |
| · Temporal Coverage: | January 1986 - December 1995 |
| · Temporal Resolution: | Monthly means |
| · Spatial Coverage: | Global |
| · Spatial Resolution: | 2.5 degree x 2.5 degree |
| · Number of Files: | One |
| · File Size: | 4.4 Megabytes (ASCII) |
| · File Name: | gpi_mth_sat-merged_mmday_asc_0186-1295 |
Data Format
Each rainfall field within the file begins with a four-digit integer
month identifier (e.g., 8601 = January 1986), followed by a 144 by 32
array of monthly rainfall estimates (in mm/day). Each value represents
the estimated areal average rainfall for a 2.5 degree by 2.5 degree
grid cell. Missing values are flagged by "9999". The data grid ranges
in latitude from 40 degree north to 40 degree south and in longitude
from 180 degree west to 180 degree east. The array structure is given
in Table 1.
Examples:
Element (1, 1) contains the mean for the grid cell centered at 38.75
degree north latitude and 1.25 degree east longitude.
Element (144, 32) contains the mean for the grid cell centered at
38.75 degree south latitude and 1.25 degree west longitude.
Table 1.
40N -------------------------- ----------------------
| | | | | | |
| (1,1) | (2,1) | (3,1) | . . . | (143,1) | (144,1) |
| | | | | | |
37.5N -------------------------- ----------------------
| | | | | | |
| (1,2) | (2,2) | (3,2) | . . . | (143,2) | (144,2) |
| | | | | | |
-------------------------- ----------------------
.
.
37.5S -------------------------- ----------------------
| | | | | | |
| (1,32 | (2,32 | (3,32)| . . . | (143,32)| (144,32)|
| | | | | | |
40S -------------------------- ----------------------
| | | | | | |
0 2.5E 5E 7.5E 5W 2.5W 0
An example program in FORTRAN to read the GPI data set is shown below:
real precip (144, 32)
character*50 infile
data infile / 'gpi_mth_sat-merged_mmday_asc_0186-1295' /
open (unit=20, file=infile, form='formatted',
+ access='sequential', status='readonly')
do 100 irec=1, 150
read (20, 1000, end=9999) iyymth, precip
write (6, *) 'Data read for time period ', iyymth
100 continue
1000 format (i4 / (144f8.2))
9999 write (6, *) 'End of file reached.'
stop
end
Arkin, P.A., 1979. The relationship between the
fractional coverage of high cloud and rainfall accumulations during GATE
over the B-array. Mon. Wea. Rev., 107, 1382-1387.
Hudlow, M.D., and V. Patterson, 1979. GATE radar
rainfall atlas. NOAA Spec. Rep., Center for Environ. Assessment Serv.,
NOAA, 155 pp.
Janowiak, J.E. and P.A. Arkin, 1991. Rainfall
variations in the tropics during 1986-1989, as estimated from
observations of cloud-top temperatures. J. Geophys. Res.,
96, Supplement, 3359-3373.
Richards, F., and P.A. Arkin, 1981. On the
relationship between satellite-observed cloud cover and precipitation.
Mon. Wea. Rev., 5, 1081-1093.
Anonymous FTP
The GPI Precipitation data set also resides online at the Goddard DAAC
anonymous FTP site and may be accessed either directly from this
document,
 Arkin and Janowiak GPI Data
- or via anonymous FTP at
- ftp daac.gsfc.nasa.gov
- login: anonymous
- password: < your internet address >
- cd http://disc.sci.gsfc.nasa.gov/data/hydrology/precip/arkin/gpcp_gpi
DAAC Help Desk:
- EOS Distributed Active Archive Center (DAAC)
- Code 610.2
- NASA Goddard Space Flight Center
- Greenbelt, Maryland 20771
- Email: daacuso@daac.gsfc.nasa.gov
- 301-614-5224 (voice)
- 301-614-5268 (fax)
Data Producers:
- The producers of this data set can be contacted as follows:
- Dr. Phillip Arkin
- Office of Global Programs
- Oceanic and Atmospheric Research
- National Oceanic and Atmospheric Administration
- Washington, D.C.
- Email: parkin@sgi17.wwb.noaa.gov
- 301-763-8317 (voice)
- 301-763-8434 (fax)
- Dr. John E. Janowiak
- Climate Analysis Center
- National Meteorological Center/National Weather Service
- National Oceanic and Atmospheric Administration
- Washington, D.C.
- Email: jsqr@sgi37.wwb.noaa.gov
- 301-763-8227 (voice)
- 301-763-8395 (fax)
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Atmos Composition
